The present invention relates, in general, to a compact resistor assembly and particularly to a compact resistor assembly for use with a multi-coil, high energy superconducting solenoid.
One form of such a solenoid comprises a plurality of superconducting coils connected in series. The solenoid is connected in circuit with a superconducting switch. A plurality of protective resistors is provided with each resistor connected in shunt with a respective one of the superconducting coils. During normal operation, with the coils, the superconducting switch and the resistors immersed in liquid helium in a cryostat, a presistent current flows through the coils. As long as the coils are in the superconducting state, no current flows through the protective shunt resistors. If one of the coils were to go normal, current would pass through the resistor in shunt therewith. With properly chosen circuit parameters, the energy passing into the circuit from the collapsing magnetic field would be primarily dissipated in the resistor shunting the coil thus protecting the quenched coil from thermal damage. If the other resistors were thermally independent of each other, each of the resistors would need to have sufficient thermal mass to absorb substantially all of the energy from the multicoil solenoid without overheating. If, however, the several resistors are in close thermal contact, the sum of all their thermal masses would be effective in absorption of energy and the total mass required would be minimized.
It is also desirable to thermally isolate the shunt resistors from the liquid helium bath in order to control the rate of pressure buildup in the cryostat during the heat up of a resistor. The assembly of shunt resistors must contain no voids that can accumulate liquid helium in order to prevent a potentially damaging pressure rise within the structure of the resistors themselves.
In order to be able to accurately monitor the field strength of the solenoid during a ramping of the current in the solenoid, the assembly of shunt resistors should produce negligible external magnetic field when current is flowing in the resistors due to the induced voltage across the coils. The ohmic values of the shunt resistors of the assembly should be proportional to the effective inductances of the individual coils that they shunt.